The invention relates to a coherent radar comprising a transmitting tube, in particular a magnetron, a modulator for driving the magnetron in order to generate and transmit HF-pulses, and a stable local oscillator (STALO) for generating an intermediate frequency signal by mixing its output signal with echo pulses caused by the transmitted pulses. The intermediate frequency signal has a frequency corresponding to the difference between the stable local oscillator frequency and the frequency of the transmitted pulses. An oscillator operating at the intermediate frequency applies an output signal to a phase sensitive detector together with the intermediate frequency signal obtained by the mixing, in order to detect the echo pulses and to generate a signal, called bipolar video, i.a. representing the relative phase position of the HF-signals in the received and the transmitted pulses. After filtering the bipolar video signal can be used, both within an unambiguous distance and at ambiguous distances, to separate targets with different radial speeds relative to the radar station, e.g. with the aim to suppress echoes from fixed objects and only indicate moving targets.
A known type of coherent radar using a magnetron as a HF-source comprises a memory for the phase of each transmitted pulse, because the phase of the magnetron has no interrelationship between successive pulses. This memory function is realized by applying a portion of the output signal of the magnetron to a mixer, where it is mixed with the output signal from the stable local oscillator. At the output of the mixer a pulse of intermediate frequency is obtained, a so-called phase locking pulse, which is led to the intermediate frequency oscillator, often called coherent oscillator (COHO), for locking the phase of the same during the transmitter pulse. The intermediate frequency oscillator then has such a stability that its phase is maintained during the listening time interval and therefore the oscillator serves as a memory for the phase of the transmitted pulse. It is apparent from the above that this memory is reset for each transmitted pulse and therefore it is not useable for incoming echoes resulting from previous transmitter pulses, so called ambiguous echoes. A receiver coherent radar of this type therefore cannot separate echoes beyond the unambiguous distance, so called second time echoes, third time echoes, etc., e.g. for suppression of strong echoes coming from large distances.
Separation of ambiguous echoes by speed filtering can be obtained if the phase and frequency of the transmitter is made adjustable. This can be realized if the output signal from the intermediate frequency oscillator, which now is made to operate continuously, is led to a mixer where it is mixed with the output signal of the stable local oscillator, which also operates continuously. The mixing product is led to a transmitter tube, where it is amplified and transmitted in pulsed shape. Thus, in this case there is continuity as regards the phase of the transmitted pulses from pulse to pulse.
As a magnetron cannot be brought to establish the same frequency and a fixed phase relationship to the mixing product of the signal from the intermediate frequency oscillator and the signal from the stable local oscillator it cannot be used in the above described type of radar.